1. Field of the Invention
This invention relates generally to suppression of stimulated Brillouin scattering which occurs in an optical fiber transmission line, and more particularly to an optical transmitter and an optical communication system which allow such suppression.
2. Description of the Related Art
In recent years, it has become possible to obtain an optical signal which is high in power and low in chirping by using an optical amplifier and an external optical modulator. When an optical signal which is high in power and coherent in this manner is to be transmitted by means of an optical fiber transmission line, stimulated Brillouin scattering (SBS) occurs in a fiber and degrades the transmission quality, and accordingly, a countermeasure against it is demanded.
An optical transmitter is known wherein bias current for inducing laser oscillations is supplied to a laser diode and modulation current is superposed with the bias current so as to directly obtain a modulated optical signal. Another optical transmitter is known wherein fixed or controlled bias current is supplied to a laser diode and continuous wave (CW) light outputted from the laser diode is modulated by an external optical modulator.
Meanwhile, it has been proposed to further frequency or phase modulate, for example, an intensity modulated optical signal in order to suppress SBS occurring in an optical fiber transmission line. The bandwidth (spectral line width) of an optical signal is expanded by frequency or phase modulation, and consequently, a threshold value with which SBS occurs increases. As a result, the SBS is suppressed.
In order to perform, for example, frequency modulation of a light beam outputted from a laser diode, it is effective to superpose a low frequency signal with bias current to be supplied to the laser diode. However, the superposition of the low frequency signal with the bias current not only modulates the frequency (wavelength) of the light beam, but also modulates the intensity (power) of the light beam. Such generation of an intensity modulated component with a low frequency signal gives rise to a problem that it degrades the transmission characteristic of the main signal and, for example, long haul transmission becomes difficult.
An optical transmitter has been proposed which includes means for suppressing an intensity modulated component by a low frequency signal described above (for example, Japanese Patent Laid-Open Application No. Heisei 7-92510). This optical transmitter includes a circuit for modulating injection current to a semiconductor laser light source with a suppression signal so that output light of the semiconductor laser light source may be phase or frequency modulated, a first external modulator for intensity modulating the intensity of the output light of the semiconductor laser light source with a signal to be transmitted, and a second external modulator for canceling an intensity modulated component by the suppression signal. However, according to this prior art, one external modulator is required for each of modulation with a signal to be transmitted and modulation for cancellation of an intensity modulated component by a suppression signal, and accordingly, this gives rise to a problem in that the construction of the optical transmitter is complicated due to the requirement for the two external modulators and peripheral circuits for them.